Hot water problem - glowworm-flexicom-cx30

[Crowdog]

I have a Glowworm Felxicom CX30. The central heating works just fine, however the hot water does not. The water does get hot but it does not stay hot, it goes cold and then hot again, repeating until displaying error F25.
I had someone to look at it and they told me it would be the heat exchange plate and slo that I need to install a filter.
So, heat exchange was taken off and clean with brick acid although I must say that it did not seem blocked - (I have taken off and repeated this cleaning 3 times in total now) - and I have fit a Magna Clean filter. The whole system has been drained after leaving in cleaning fluid and I have put some system inhibitor in.
Despite all I've done, I still have the same fault - hot then cold 3 time before error F25 come up.
I put the boiler in state Mode and left the hot tap running and below is the sequence it produces. I'm hoping someone can help with a proper diagnosis before we decide that a new boiler is the only option (rather not just before Christmas but we can't keep on with the current situation)

Hot water turned on - time below is when the state number is first displayed:
13 ignition
14 burner on - 9 seconds
16 fan overrun
53 time: temperature difference flow return sensor too high - 30 seconds
16 fan overrun
10 dhw demand - 40 seconds
53 waiting time: temperature difference flow return sensor too high
13 ignition - 1 min 10 secs
14 burner on
98 flow return - sensor check is running demand is blocked - 1 min 28 secs
97 water pressure sensor check is running demand is blocked - 2 min 30 secs
13 ignition - 2 min 38 secs
14 burner on - 2 min 58 secs
53 waiting time: temperature difference flow return sensor too high - 3 min 38 secs
98 flow return - sensor check is running demand is blocked - 6 min 39 secs
F25 - 6 min 48 secs

Any help would be massively appreciated.
Thanks in advance,
Steve

*edit to say that rads have been bled and none of them are holding air

 

[Riley]

Afraid you need a gas safe engineer in as you can’t be working in your boiler it’s illegal

 

[Crowdog]

I appreciate the concern and should say that I am in no way touching the combustion side of the boiler and would not dream of doing so. Removing the front panel does not require me to be gas safe AFAIK although I'm prepared to be told otherwise. We did have someone out and he told me to fit the filter and clean out the heat exchange plate, which I have done already.

 

[Riley]

Cover creates a combustion seal so you need to be able to test its back on properly

 

[Crowdog]

OK, thanks for the knowledge. Annoying how the guy from SSE didn't say that when he came out to fix it! My dilemma is affording a new boiler or getting someone to look at the fault again. 2 engineers have both suggested the same thing which it does not seem to be, hence why I'm here really. Could anyone at least give a thought on a possible fault so I can relay the information to get a cost on fixing?
A family of 4 not being able to have a proper shower is hard work and buying a new boiler just before Christmas is awful timing.

 

[Riley]

We can’t give you tips as that kind of negates our gas advice policy. If you have lost faith in independent engineers then contact glowworm and they offer a fixed price callout and will replace any parts of the boiler required with the exception of the main heat exchanger

 

[Riley]

Is it definitely the boiler though. Does the hot cold issue happen at other outlets

 

[AMGasServices]

Get Glowworm out to do a one off repair.

 

[Crowdog]

Thanks for the advice. I guess all I need to know if it's the boiler at fault because after being told otherwise twice, I wouldn't want to pay for someone to come out and tell me they can't do anything because it's not the boiler.
To answer and earlier question, the issue of hot cold happens regardless of the tap used or if its the shower in use.
Thanks again.

 

[Riley]

Then I would say boiler problem